(1) Field of the Invention:
This invention relates to resin compositions for providing novel hard polyvinyl chloride (which may hereinafter be abbreviated as PVC) resin open-cell foams which are useful in a wide variety of application fields such as underdrain pipes, various aeration pipes, slope draining piles, various air filters, solid-gas separating materials, solid-liquid separating materials, materials for jetting or bubbling gas into liquid, deodorant bases and fragrance carriers, and also to the open-cell foams thus obtained from the compositions.
(2) Description of the Prior Art:
As open-cell foams made of a synthetic resin, flexible urethane foams obtained by a reaction between a saturated polyester and a polyether have been well known to date. Besides, there are polyvinyl alcohol (which may hereinafter abbreviated as "PVA") base sponges obtained by adding .alpha.-starch as a cell-forming agent to PVA and subsequent to conversion into a formal, washing away the starch with water.
As known processes for obtaining open-cell PVC resin foams, there are those disclosed, for example, in Japanese Patent Publication No. 21898/1978, U.S. Pat. No. 4,226,943, and Japanese Patent Publication Nos. 50662/1983, 25369/1983 and 58370/1983.
The invention described in Japanese Patent Publication No. 21898/1978 relates to a process for producing an open-cell foam by forming a mixture, which contains at least one polymer of emulsion homopolymer and emulsion copolymers of vinyl chloride, a plasticizer and a foaming agent, into a sheet-like shape, followed by heating, characterized in that the mixture is added with at least one wax selected from paraffin waxes having a carbon number of 20-80 and ester type waxes consisting of monohydric and polyhydric alcohol esters of higher aliphatic monobasic acids and having 20-80 carbon atoms, said at least one wax having substantially no miscibility with said at least one polymer.
The invention described in U.S. Pat. No. 4,226,943 is directed to a process for the production of a sheet-like open-cell foam of a PVC base resin, which comprises the steps of forming a foamable PVC resin composition into a sheet and then heating the sheet to a temperature at which a foaming agent is decomposed in the sheet. The composition is composed of the following components:
(1) 100 phr of a PVC base resin; PA1 (2) 30-150 phr of a plasticizer; PA1 (3) 1-8 phr of a synthetic hydrocarbon wax, which has been prepared in the course of a process selected from depolymerization of polyethylene, polymerization of ethylene and Fisher-Tropsch synthesis of coal gas and has a molecular weight of 1,500-7,000, a softening point of 90.degree.-135.degree. C. and a particle size of 10-100 .mu.m; PA1 (4) 1-5 phr of an alkali metal-Zn type stabilizer; and PA1 (5) 2-10 phr of a heat-decomposable organic foaming agent at least 60% of which is composed of azodicarbonamide. PA1 (1) a wax (in the case of Japanese Patent Publication No. 21898/1978); or PA1 (2) a wax and an alkali metal-Zn type stabilizer (in the case of U.S. Pat. No. 4,226,943);
Although the PVC base resin employed in the latter invention is not specifically described as an "emulsion polymer", it is clearly recited in its descriptive portion that a PVC base resin produced by emulsion polymerization is used. Further, a PVC base resin produced by emulsion polymerization was used in each of the Examples. It is hence apparent that like the former invention, the latter invention is also directed to a process for producing an open-cell foam from an emulsion polymer resin by a coating technique.
The features of these inventions may be summarized as follows:
(1) A PVC base resin produced by emulsion polymerization is used.
(2) The product is a soft sheet-like product containing a plasticizer in a large amount.
(3) A heat-decomposable organic foaming agent is used
(4) As essential components, are added:
(5) Other necessary components are added to form a past-like coating formulation.
(6) The above coating formulation is applied to a base, followed by its gelation into a sheet.
(7) The sheet is heated in a foam oven to expand same into an open-cell foam.
The three inventions described respectively in Japanese Patent Publication Nos. 50662/1983, 25369/1983 and 58370/1983 have been assigned commonly to the present assignee and constitute basis for the present invention.
The invention described in Japanese Patent Publication No. 50662/1983 relates to a process for the production of an open-cell foam, which comprises heating and kneading a resin mixture prepared by adding, to a PVC base resin useful generally as a plastisol, a plasticizer, a mixed or composite stabilizer formed in combination of an organic stabilizer containing a Group-I metal of the periodic table and another organic stabilizer containing zinc and one or more other metals, powder of a heat-decomposable organic foaming agent employed usually as a foaming agent for the formation of open cells, such as azodicarbonamide, and if necessary, one or more other additives; processing the thus-kneaded mixture into a sheet by a suitable technique such as calendering or extrusion; and then heating the sheet to the decomposition temperature of a foaming agent.
The invention described in Japanese Patent Publication No. 25369/1983 is directed to a process for the production of an open-cell foam, which comprises heating and kneading a resin mixture, processing the thus-kneaded resin mixture into a sheet by a suitable technique such as calendering or extrusion, and then feeding the sheet to a foam oven to heat and expand same. The resin mixture is prepared by adding, to a suspension-polymerized general-purpose PVC base resin as a principal component, a combination of an organic stabilizer containing a Group-I metal of the periodic table and another organic stabilizer containing a Group-II metal of the periodic table, or an organic stabilizer containing both Group-I and Group-II metals of the periodic table; a combination of an anionic surfactant and a non-ionic surfactant, or a combination of an anionic surfactant and a low polymerization-degree polyolefin or wax; and further a plasticizer; a heat-decomposable organic foaming agent employed generally for the production of open-cell foams such as azodicarbonamide; and other additives such as filler as needed.
The invention of Japanese Patent Publication No. 58370/1983 is directed to a process for the production of an open-cell foam, which comprises heating and kneading a mixture of a PVC-base resin composition (containing a PVC-base resin, plasticizer, stabilizer, filler, etc.), 4,4'-oxybis(benzenehydrazide) as a foaming agent and an anionic surfactant, forming the thus-kneaded mixture into a sheet and then heating and expanding the sheet.
The technical concept common to these three invention may be summarized as follows:
(1) As a processing method, a sheet is formed first of all by calendering (or extrusion) and the sheet is subsequently heated in a foam oven to foam the sheet.
(2) As a PVC-base resin, is used that produced by suspension polymerization (an emulsion-polymerized resin is used in Japanese Patent Publication No. 50662/1983 only).
(3) The foam is a soft PVC-base resin product in which 55-80 phr of a plasticizer is contained.
(4) As a stabilizer, is used a mixture of a stabilizer containing a Group-I metal of the periodic table and another stabilizer containing a Group-II metal of the periodic table, or a composite stabilizer containing both Group-I and Group-II metals of the periodic table.
(5) A surfactant is used (no surfactant is employed in Japanese Patent Publication No. 50662/1983).
The products obtained in accordance with these inventions are suitable for use as interior finishes for automotive vehicles and upholsteries. Accordingly, many of such products are used in a form laminated with a backing such as knitted fabric, plain weave fabric or non-woven fabric. For such applications, the above-mentioned inventions including those of Japanese Patent Publication No. 21898/1978 and U.S. Pat. No. 4,226,943 are extremely effective. They are however not applicable at all for the above-mentioned application field, namely, for the production of hard PVC-base resin products, especially, formed articles such as pipe-like products and plate-like products.
On the other hand, PVA sponge is hard and remains in a state excellent in strength so long as it is dry. It is however softened upon exposure to water or moisture and is hence unapplicable to the above application fields. A cumbersome reaction such as conversion into an acetal is additionally required in order to convert PVA sponge into a state that it is no longer softened with water. Further, polyurethane foams are obtained only as irregular open-cell foams and are thus inappropriate for the above application fields.
In order to produce an open-cell foam of a hard PVC-base resin, it is not suitable to add a plasticizer in a large amount as a first requisite. Even if added (it is not added to obtain a soft foam but is incorporated as a processing aid so as to facilitate the extrusion processing), the plasticizer is limited to a very small amount of 5 phr or less. The thermal and rheological behavior of the composition of raw materials for the open-cell foam of the hard PVC-base resin upon its processing is therefore different totally from that of a raw material composition for a soft PVC-base resin product.
Pipes, plate-like products, rod-like products and other like products are formed by extrusion. In this case, the formation of open cells in a formed article must be performed at a time point that the composition of raw materials has just passed through a die of an extruder. It is therefore impossible to apply conventional processes, namely, to form the raw material composition into a sheet beforehand by calendering or extrusion and then to heat the sheet again so as to foam it into an open-cell foam.
In the case of an open-cell foam composed of a hard PVC-base resin, its production process requires such steps that the resin is heated and kneaded in an extruder to gel and melt the resin and at the same time to decompose and gasify a foaming agent, the resin is caused to expand concurrently with the completion of its passage through a die of the extruder, and the open-cell foam is formed by the expansion. These process requirements cannot however be fulfilled by any of the above-described conventional inventions no matter they are applied singly or in combination.